Error when sending emails using Mailjet and promises

Question

I get an error when I'd like to sen emails using promises. I'm using promises because I have 889 emails to send and with Mailjet API I can send 50 emails per calls.

My code :

function arraySend(array, locals) {
  var mailjetConnect = mailjet.connect(****username, ****password),
      chunkedArrayMessages = _.chunk(array, 50),
      promises;

  if (process.env.NODE_ENV !== 'dev') {
    promises = chunkedArrayMessages.map((messages) => {
      return mailjetConnect.post('send', { version: 'v3.1' }).request({ Messages: messages });
    });
    return Promise.all(promises).then(function() {
      return Promise.resolve();
    }).catch(function(err) {
      return Promise.reject(err);
    });
  } else {
    return Promise.resolve();
  }
}

Error message :

UnhandledPromiseRejectionWarning: Error: Unsuccessful ... Mailjet

Mailjet API : https://dev.mailjet.com/email/reference/send-emails/#v3_1_post_send